Behavior of hydrogen-enriched non-premixed swirled natural gas flames

The effects of hydrogen addition on a lean non-premixed natural gas swirl-stabilized flame were investigated. Fuel mixtures containing a volumetric fraction of hydrogen ranging from 0% up to 100% were burnt at ambient pressure with swirled air within a quartz chamber in a co-flow configuration. Tests were carried out keeping the sum of the volumetric fuels flow rate constant; thus, the fuel mixture mass flow rate, input thermal power and equivalence ratio decreased as hydrogen fraction was increased. The use of hydrogen-enriched natural gas mixtures allowed the burner to operate at overall leaner stable conditions as compared to the case of burning natural gas only; however, an increase in soot, NOx and CO has also been observed. The flame structure was analyzed by still-color photographs and laser sheet visualization; laser Doppler velocimetry (LDV) and thermocouples were used to analyze the flow field and temperature distributions. Measurements of CO and NOx were obtained by probing the exhaust gases and using gas analyzers, while soot was directly identified by the yellow luminosity in the flame photographs and estimated through spectral analysis of spontaneous flame emission. The present results revealed that hydrogen addition produced: (1) a shorter and narrowed blue flame located closer to the burner head, (2) a central highly luminous yellow plume extending above the visible blue zone, (3) a deeper fuel jet penetration inside the recirculating bubble, and (4) a monotonic increase of both CO and NO., emissions for H-2 fractions ranging front 0% up to 80%. The latter behavior may be due to quenching of CO oxidation, related to the reduction in the size of the reaction zone, while the temperature increase observed near the flame front and close to the burner head promote the thermal NOx production. The experimental results revealed that hydrogen addition extended the stability limits of a conventional natural gas nonpremixed burner and evidenced a significant change in both the flame structure and the flow field. Improved fuel mixture injection strategies should be explored to improve mixing and minimize pollutant production, without affecting flame stability. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.